Efficient high torque continuously variable transmission

ABSTRACT

Continuously variable transmission includes a set of pulleys, each pulley comprising two discs arranged relatively axially moveable on a central pulley shaft for clamping a transmission belt. The belt includes a continuous array of transverse elements, each having lateral pulley contact surfaces for contacting the belt contact surface of each of the pulley discs, and being provided slideably on an endless tensile element of the belt. Each pulley has a mechanical stiffness as incorporated in the transmission and a stiffness related feature of a pulley when expressed as a parameter Sag that indicates an amount of radial displacement of the belt between the discs in the transmission, occurring in response to imposing a maximum amount of axial force Fax during operation of the transmission, relative to a radial position of the belt in an unloaded state of the transmission, amounts to less than 1.2 mm but more than 0.5 mm.

The present invention relates to a continuous variable transmission asdefined in the preamble of claim 1.

Such transmissions are generally known in the art for example fromEP-B1-0.777.069 and are used for transmitting mechanical power in amotor vehicle from an engine to a load, i.e. the driven wheels, at acontinuously variable torque ratio. They are typically designed for, andcommercially operate at a cone angle of the pulley discs for contactingthe assembled transmission belt of eleven degrees as taken relative toan imaginary radial line through the axis of rotation the relevantpulley, i.e. the centre line of a pulley shaft. The pulleys typicallyhave a diameter between one to two hundred mm.

Yet little is, however, known in the art about the criteria, to beapplied at designing the pulleys. Thus the principle design of a pulleyin practice hardly deviates from the pulley designs devised in the earlydays of belt type continuous variable transmissions. It appears inpractice that when a known pulley design is modified to even a slightlydifferent cone angle, or to cope with an increased torque level, anotherwise properly functioning commercialised pulley may perform badly,particularly in terms of transmission efficiency. This means that anunexpectedly large amount of mechanical power is dissipated duringoperation of such a modified transmission. It is generally accepted thatthe stiffness of pulleys, alternatively denoted, the amount of axialpulley disc deflection therein at the radial outer edge of the discs,influences the performance of a transmission. No clarity exists,however, which relations validly define pulley stiffness, or how anyconceivable relations could be explained when valid.

Apart from said cone angle the known transmission designs feature acertain stiffness based on the material applied in the pulleys, and acertain type of bearing, a certain amount of play at various fittingsand bearings, all of which are known to influence the performance of thetransmission. The direct relation between any of such features or theeffect of their mutual interaction and the performance of thetransmission is presently not exactly known nor apparently in control bydesign rules. Although the mentioned design characteristics per se,including e.g. the internal stiffness of a pulley disc are well withinreach of a skilled man to be controlled to a desired level of quality,e.g. by applying a certain material composition or thickness, it isgenerally unclear how these features may individually or in common beapplied for influencing the performance of a transmission. For examplein relation to the transmission efficiency, it was revealed in practicethat contemporary and proto-type transmissions that are designed andintended to be commercialised for applications having increased powerand torque levels compared to currently commercialised transmissions byupscaling existing transmission designs, surprisingly do not reach theefficiency that may be expected on the basis of transmissionefficiencies realised with these existing transmission designs. Thisphenomenon revealed itself especially at prototype transmissionsfeaturing a relative small cone angle as may be preferred in certainapplications, i.e. smaller than 11 degrees or transmission designsintended to cope with torque levels of 250 Nm or more.

Thus, it is an object of the invention to provide transmissions and agenerally applicable design rule therefor in particular related to thepulleys thereof, which guarantees on the one hand that the transmissionwill operate safely at a proper level of efficiency without laying anundue burden on the design and production of other components applied insuch transmission, and thus also accommodating the tendency towardsapplying very high axial clamping forces and/or smaller cone angles atwhich a belt and a pulley co-operate in a transmission.

According to the invention, such is realised when the said transmissionfurther shows the characterising features of claim 1. In accordance withthe idea underlying the invention and test results, it was found that abelt applied in a transmission not meeting the criteria developed inaccordance with the claim, suffers from what is denoted extreme spiralrunning of the transmission belt within a pulley, i.e. a relativelylarge deviation of the theoretical circular trajectory. So as to come toa generally applicable rule for testing a pulley design this phenomenonhas in accordance with the invention been tried to be captured in asingle parameter by modelling, which parameter is referred to as Sag.

As to the assessment of spiral running of the transmission belt at ornear the largest possible radius of curvature of the belt when runningbetween the pulley discs, where it is the most often located at thepulley associated with said source of mechanical power during normaloperation of the transmission, the actual running radius that locallydescribes the curved running path of the belt was found to sometimeschange for more than 2 mm for pulleys of contemporary design that aretypically applied in automotive applications. Accordingly, in such caseeach longitudinal section of the belt moves somewhat radially inwardover the pulley discs, i.e. shows a slipping motion with respect to thepulley discs, as it passes along the said curved trajectory therebetween. Also, besides not conforming to the theoretical circulartrajectory, the belt's trajectory was found to deviate from a shape thatmay be expected based on pulley disc deformation considerations such asare known from EP-A-0.962.679, wherein it is taught that as a result ofthe transmission belt being present between the pulley discs only over apart of the circumference thereof, the spacing between the pulley discsincreases and subsequently decreases along the theoretical circularlycurved trajectory of the transmission belt. From these known deformationcharacteristics it may be expected that the trajectory of the belt partrunning between the pulley discs would consist of a slight radiallyinward movement of the said longitudinal belt section at enteringbetween the discs of a pulley and a corresponding radial outwardmovement of the belt at exiting from running between the pulley discs.

Such observed shape of the curved trajectory and such a large absolutechange in the running radius of the transmission belt, thus was found tosignificantly differ from the form and amount thereof that may beexpected beforehand. These phenomenon imply that the belt iscontinuously slipping between the pulley discs at least in radialdirection. Since the assembled belt transmission belt that is presentlyunder consideration is composed of a multitude of relatively thintransverse element, which are slideably mounted on an endless band, eachtransverse element may at least to some extent move freely with respectto neighbouring elements. Therefore, it may be concluded that such beltdesign is more susceptible to slip compared to longitudinally rigid beltdesigns such as a chain or a continuous V-belt. Thus the assembledtransmission belt in combination with a large clamping force by whichthe belt is clamped between the said pulley discs and that is requiredin modern transmission creates much larger losses in efficiency of thebelt in modified transmissions than those which would be anticipated.Especially at transmissions featuring a relative small cone angle, i.e.smaller than the 11 degrees that is presently applied in commerciallyavailable transmission, the spiral running effect at pulleys having amechanical stiffness comparable with known pulley designs surprisinglyappeared to be larger still, having the above explained adverse effecton the torque transmission efficiency even at the same or similar levelof the clamping force.

According to the invention an important consequence of the observedspiral running of the belt, i.e. not following a circular trajectory nora symmetrical path that comprises two sections each the mirrored imageof the other, is that the normal force between belt and pulley discs isnot distributed evenly along said curved trajectory, but rather appearsto continuously increase. The maximum level of such force will thus belarger than what was previously thought. This effect becomes morepronounced at higher levels of the clamping force, i.e. at higher torquelevels to be transmitted by the transmission.

It could be concluded from the above, that designing the pulleys to havea higher stiffness using well known constructional measures would be anadequate and simple measure to reduce the observed radial slip therebyimproving transmission efficiency. However, according to the invention,it was further established in tests that a transmission of such astiffer design would not necessarily function to satisfaction. It ishypothesised that this is caused by mechanism involved in shifting ofthe transmission, i.e. at a desired radial movement of the belt over itsentire part located between the pulley discs. It appears that suchradial movement is not only the result of the elements being forced tomove, i.e. slip, in radial direction along the pulley disc surface, butis also supported by the deformation of the pulley discs as describedabove, which assists in the desired relative radial movement betweenindividual transverse elements and the pulley discs. Accordingly, it isconsidered that increasing the stiffness of the pulleys does notautomatically render an efficient and functional, transmission per se.

Apart from the identification and impact of the above described spiralrunning on the transmission operations, the invention features a newcriterion and allowable value therein for determining the quality ofperformance of a transmission and in particular the torque transmissionefficiency thereof. In this criterion, a maximum allowable amount ofaxial deflection of the pulley discs at a radially outermost edgethereof occurring during operation of the transmission is related bothto the maximum level of the clamping force applied and to the cone-angleof the transmission.

The invention will now further be explained by way of examples given inthe accompanying drawing wherein:

FIG. 1 is a schematic illustration of a side elevation of thecontinuously variable transmission to which the present inventionrelates provided with pulleys and a transmission belt;

FIG. 2 is a schematic representation of the transmission belt comprisinga tensile means whereon a large number of transverse elements aremounted. The figure includes an enlargement of the arrangement showingthe tensile means to comprise a number of bands as well as theinteraction between a band and a saddle surface of the element;

FIG. 3 is a figurative representation of forces exerted by the pulley onthe transmission belt as represented in a schematic transverse sectionof the belt when wedged between the discs of the pulley;

FIG. 4 is an illustration of possible trajectory of the transmissionbelt showing the spiral running on the basis of typical measurement datarelated to this phenomenon;

FIG. 5 schematically illustrates the effect of axial deflection of thepulley disc at different cone angles;

FIG. 6 is a plot indicating the transmission efficiency in relation tothe amount of possible radial slip of the transmission belt in itscurved trajectory part;

FIG. 7 is schematic representation of a pulley suited for FEMcalculations;

FIG. 8 is a graph indicating the relation between cone angle and theaxial deflection according to the invention;

FIG. 9 schematically illustrates a second measure according to theinvention for improving the torque transmission efficiency of thetransmission;

FIG. 10 schematically illustrates a third measure according to theinvention for improving the torque transmission efficiency of thetransmission.

FIG. 1 schematically shows a continuous variable transmission (CVT) witha transmission belt 1 which is made up of a tensile means 2 in the formof nested endless thin bands, and on which there are mounted an array ofcross elements 3, alternatively denoted transverse elements, which mayslide freely over the tensile means 2. The trajectory of the belt 1shows a curved part where it runs on the pulleys 4, 5, i.e. betweenfrusto-conical pulleys discs thereof. As the pulley discs of one pulleyare urged towards each other, the belt 1 is locally forced to shiftradially outward, whereby a radius of curvature of the belt's localtrajectory, i.e. its so called running radius, increases. At the sametime, due to the virtually constant circumference length of the belt 1,a belt part between the discs of the other pulley shifts radially inwardwhereby its urges the pulley discs apart and the local running radiusdecreases. The running radius of the belt 1 in the respective pulleys 4and 5 determines the torque transmission ratio between the pulleys,which may be continuously varied by the said radial shifting of the belt1. Such a continuous variable transmission is known per se. A typicalthickness of the said band ranges from 0.15 to 0.25 mm, whereas atypical width of the belt 1 ranges from 15 to 35 millimeters at atypical circumference length thereof that range from 50 to 100 cm. Forreasons of economical production and for preventing technicalcomplexity, usually only one pulley disc is arranged axially movable toallow said radial shifting, whereas the other disc is arranged axiallyfixed with respect to the pulley shaft.

FIG. 2 depicts a longitudinal cross section of the belt 1, showing afront view of the transverse element 3 and a cross section of thetensile means 2. On its lateral sides the cross element 3 is providedwith pulley contact surfaces 6 by which it may rests against the beltcontact surface of the frusto-conical pulley discs of a pulley. Theelement 3 is further provided with so called saddle surfaces 7 that mayinteract with the tensile means 2, in particular in a part of the belt's1 trajectory curved in longitudinal direction where the belt runsclamped between the discs of a pulley.

FIG. 3 schematically represents the forces acting between a belt and thepulley within a transmission. The normal force Fn, acting perpendicularto the local plane of contact between the pulley 4 and the belt 1 iscommonly realised by the application of a hydraulic pressure on theaxially outer face of at least one disc of the pulley 4. To this end, apiston/cylinder assembly is accommodated in a known manner wherebyeither the cylinder is fixed to the pulley shaft and the piston is fixedto a pulley disc 16 that is an axially movable with respect to thepulley shaft 14 or vice versa. The cylinder is in turn fed by ahydraulic pump, usually made part of the transmission. By the exertedhydraulic pressure the moveable disc 16 is urged in a direction towardsthe respective other disc 15 wedging the belt in between said discs 15,16, which results in said normal or reaction force Fn. In dependence ofthe relevant cone angle λ of a pulley disc 15, 16 the normal force Fnmay mathematically be split up in an axial directed force component Fax,alternatively denoting the belt clamping force and a radial outwarddirected component Fy. The sum of the axial forces Fax acting on alltransverse elements 3 in contact with a pulley 4, 5 and the localcoefficient of friction μ determine the force transferable between thebelt 1 and the pulley 4 by means of friction, which in combination withthe local running radius of the belt determines the maximum amount oftorque that may be put on the pulley shaft by the belt 1, or vice versawithout mutual slip in a tangential direction occurring there between.In this respect, when present such tangential usually occurs at thepulley 4, 5 having the smallest amount of contacting transverse elements3, i.e. where the running radius, i.e. the radius of curvature of thesaid longitudinally curved trajectory part, of the belt 1 is smallest.Moreover, by increasing the clamping force for one of the pulleys 4, 5with respect to the other, the elements 3 clamped between the discs ofsaid one pulley 4, 5 may be urged radial outwardly. Otherwise theelements 3 may move radial inwardly when the clamping force of this onepulley 4, 5 is reduced in favour of the clamping force of the otherpulley 4, 5.

Since at both pulleys 4, 5 the belt 1 receives a radially outwarddirected force Fy that urges the elements 3 outward, the tensile means 2is set under a certain tension through contact with the saddle surfaces7 of the elements. Hereby the tensile means 2 performs a function inmaintaining the integrity of the belt 1. The saddle surfaces 7 areshaped slightly convex in transverse direction so as to promote acentred tracking of the tensile means 2 on the said saddle surfaces 7during the numerous number of revolutions that a belt 1 undergoes attransmitting power from one pulley 4, 5 to the other during operation ofthe transmission.

Also in FIG. 3 the transmission is shown in a condition near the socalled OD condition in which condition the belt 1 at a drive pulley 5,i.e. the pulley 5 that provides a friction force on the belt 1 drivingit into a tangential direction, has assumed a largest possible radialposition between the discs of the pulley 15, 16. The OD condition is themost often occurring condition during operation of the transmission.From the illustration of FIG. 3 it may be taken that at an axiallydeflection of the pulley discs, i.e. a local bending apart of the pulleydisc or discs under the influence the reaction force to the saidclamping force Fax, the belt 1 locally assumes a radial position whichis displaced by an amount Sag radially inwardly. Certainly at the outeredge of the discs, each pulley design is to a larger or lesser extendprone to such axial deflection Dax under the exertion of the axial forceFax due to its limited stiffness, which causes the belt 1 to assume amore radially inward running path under the influence of the tensilemeans 2 acting on the saddle surfaces 7, while it passes it's trajectorywithin the pulley 4, 5. This effect, measured with probes accuratelysensing a radial disposition of the belt 1, is in the present inventioncalled spiral running of the belt.

FIG. 4 is a schematic illustration of the running path of a belt in atransmission as discovered by the research underlying the invention,wherein the radial inward displacement of the belt 1 has beenconsiderably exaggerated. It shows that between the discs of the pulley5 this path deviates from the ideal running path where it running pathcorresponds to a circular trajectory of certain radius. It also deviatesfrom the path that may be expected based on known pulley deformation asa result of the fact the belt 1 is only present between the said discsover a part of their circumference. In fact the curved part of thebelt's trajectory as found in accordance with the invention, in mostcases, shows a steady, virtually none reversing decline of the runningradius of a belt. As mentioned earlier the amount of radial displacementof the belt 1 in one such curved part was found to be unexpectedlylarge. Moreover, from the shape of the path it may be concluded that thesaid normal force Fn increases in the direction of movement of the belt1, from which it may be concluded that the maximum normal force Fnoccurring, is considerably larger than its nominal or average value thatmay be calculated based on the pressure exerted in the said pressurechamber and the total surface area of the contact faces 6 of theelements 3 present between the pulley discs at any one time.

Such large amount of radial inward motion, or radial slip of the belt 1at such large normal force results in a considerable loss oftransmission efficiency. All the more, since radial slip inevitably alsoleads to tangential slip because the transmission belt 1 has alongitudinal speed that can correspond to the tangential speed of thepulley disc only at a single running radius, whereas due to the saidspiral running the belt 1 in fact is in contact with the pulley discs atseveral radii simultaneously at any one instance. According to theinvention, the transmission efficiency is particularly adverselyaffected in case of transmissions operating with a cone angle λ smallerthan 11 degrees such as those that are currently being developed forfuture commercial applications.

FIG. 5 illustrates the influence of the cone angle λ on this phenomenonunderlying the invention. FIG. 5 shows that for a given axial deflectionDax of the pulley discs, which deflection is of course determined by thestiffness of a pulley and the clamping force, the amount of radialinward displacement of the belt Sag that results therefrom considerablyincreases as the cone angle λ decreases. In accordance with theinvention the phenomenon is taken into account by dividing the axialdeflection Dax of the moveable disc by the tangent of the cone angle λ.In FIG. 5 the lines 9 to 11 represent different orientations of the beltcontacting surface of a pulley discs, for example the left disc of thepulley 4 depicted in FIG. 3. Lines 8 and 9 relate, to such contactingsurface incorporated under a relatively small cone angle λ, e.g. 5degrees, whilst lines 10 and 11 relate to such contacting surfaceincorporated under the typically applied cone angle λ of about 11degrees. It is demonstrated that at an identical amount of axialdeflection Dax of both surfaces 8 and 10, assuming the posture denotedas 9 and 11 respectively, the resulting radial movement of the belt Sag2for the contacting surface 8, 9 oriented at the small cone angle λ isconsiderably larger than that Sag1 for the contacting surface 10, 11oriented at the larger cone angle λ, whereby the Sag parameter may becalculated by diving the axial deflection Dax by the tangent of the coneangle λ.

An illustration of the predictive capacity of the invented criterion,alternatively denoted the Sag parameter is given by FIG. 6. The graph inFIG. 6 shows the loss in the torque transmission efficiency for severalvalues of Sag between 0 and 3 mm. It appears that the predictedefficiency of the CVT-transmission designed according to this parameterdrops significantly, as demonstrated by the circular dots in the graph,which relate to prototype and commercially applied pulley designs ofdifferent mechanical stiffness and different cone angle λ. It isremarked that it is assumed that efficiency loss in load bearings 15, 18and losses in the belt 1 itself remain equal between infinitely stiffand practical pulley designs. It is also assumed that any otherefficiency losses may be subscribed to the value of Sag.

According to the invention, instead of measuring the radial inwarddisplacement of the belt, i.e. the Sag parameter, the value thereof maybe accurately and conveniently approximated by a FEM calculation method,which is defined below. Such method allows the validation of a newpulley design with respect to the phenomenon of torque transmissionefficiency, even before a prototype has been manufactured. The method isbased on a generally applicable and available computerised andstandardised FEM calculations, departing from a set of data in which allvalues of X, Y and Z dimensions of the geometrical features of thepulley shaft 14 and the fixed disc 15 and the net forces exerted thereonby the transmission belt, the movable pulley disc and the shaftbearings, as discussed in the below with reference to FIG. 7. TheX-direction is the axial direction, which also is the axis of rotationof the pulley 4, whereas the Y and Z directions are orientedperpendicular thereto and to each other. The bearings 12, 13 of thepulley shaft 14 are considered and introduced in the FEM calculation asa so-called multi-point constraint (MPC), i.e. a mesh node located on acentre line of the shaft 14 corresponding to the halfway point of theaxial dimension of the bearing, which node is surrounded by nodes lyingon the radial outer surface of the shaft 14 over the said axialdimension of the bearing 12, 13, which latter nodes follow anytranslation and rotation of the said central node. One bearing 13 isfixed in the Y- or Z-directions but allows translations in theX-direction, whereas the other bearing 12 is fixed in all of the X, Yand Z directions. The bearings 12, 13 are further considered fixed withregard to rotation about the X-direction, whereas rotations about the Y-or Z-directions are allowed.

According to the invention, for the purpose of calculating the axialdeflection of the fixed pulley disc 15, the clamping force and thereaction force are considered to be equally distributed over all thetransverse elements 3 that are clamped between the said discs 15, 16.The clamping forces exerted on the belt and the reaction forceaccompanying the clamping force are in this standardised FEM calculationaccording to the invention applied as pressures.

The method further deals with bending forces on the shaft 14 by atilting motion of the movable disc 16 and its sleeve 19 of the shaft 14that occurs in practice and that originates from the reaction forceexerted by the belt 1 on the movable disc, which is schematicallyindicated in FIG. 7 to a considerably exaggerated extend. In thecalculation method such bending forces to be equally distributed overhalf of the shaft's circumference 20 at one axial end 21 thereof closestto the fixed disc 15 and over the opposite half of the shaftscircumference 17 at the opposite axial end 18 thereof and also equallydistributed over the axial extend of such circumferences that conformsto the area where the movable disc 16 and the shaft 14 would contactduring operation under the influence of the said reaction force. It wasfound that an area of about 3 mm that starts about 0.5 mm inward fromthe precise axial position on the shaft of the said axial ends 21, 18 isa good approximation thereof. The axial deflection Dax calculated withthe thus created FEM calculation method also includes the contributionof a bending of the pulley shaft 14.

The FEM elements used in the calculation method are 2D mesh andso-called QUAD4. The size is 5 mm and approximately 1000 elements areused. The aspect ratio is smaller than 5. 3D elements are created byrotating 2D elements 24 times over 15 degrees. Of course, only halve ofthe 3D model is actually required (i.e. 12 times 15 degrees) because theconstruction can be divided in to two mutually symmetrical halves. Thepulley material is specified by its E-modulus and Poisson's ratio, whichfor common steel may be set at 210.000 N/mm² and 0.3 respectively.

With the above-described FEM calculation method the axial deflection ofthe pulley discs may be easily determined and the Sag parameter isobtained by dividing the said axial deflection Dax by the tangent of thecone angle λ. Generally the most extreme Sag value for a transmission isobtained for the drive pulley with the transmission operating inOD-condition at a maximum transmission torque level in such OD-conditionand with a corresponding maximum clamping force applied. It may beappreciated that the criterion Sag is universal in that it applies forany pulley design, irrespective of the cone angle. It is quiteadvantageous in that it allows a check of a pulley design as to aprediction of its level of performance in reality, before the pulleyeven has been build, by performing a relatively simple FEM calculationalong the directions provided by this invention.

In FIG. 8, as a consequence of the earlier mentioned insight underlyingthe invention, a relation between a practically tolerable maximum amountof axial deflection Dax and the value of the cone angle λ isrepresented. The radial lowering of a belt in a pulley Sag is inaccordance with the invention calculated from these parameters bydividing the amount of axial deflection Dax, as calculated from thestandardised FEM model of the pulley, by the tangent of the cone angle,i.e. by tan(λ).

It was found in practice in accordance with the underlying research andanalysis that proper functioning of a transmission can with goodcertainty be found at Sag values below 1.2 mm. According to theinvention it is, however, also to be considered preferable that the Sagparameter is larger than 0.5 mm, because according to the invention, atleast some radial slip is required for the reliable operation of thetransmission, e.g. for realising fast and smooth shifting of thetransmission ratio by a desired radial movement of the belt. Moreover,at least some deformation of the pulley discs and thus a notional valueof the Sag parameter may even be considered an advantage, because theforces between the elements and the discs are then distributed over acertain area whereby the contact stresses are kept within design limitsand whereby the elements assume a well defined posture between thepulley discs. Said Sag value of 0.5 mm was found to be a suitable lowerboundary. In FIG. 8, the curves 22 and 23 are respectively draw for Sagequals 1.2 mm and Sag equals 0.5 mm. Between the said curves 22 and 23an area of applicable values for the axial deflection Dax of a pulleydisc in combination with values for the cone angle λ is defined.Transmissions designed with the above-mentioned constraints have optimumtorque transmission efficiency even when transmitting a relatively hightorque and/or when incorporating a relatively small cone angle λ, i.e.smaller than 11°.

According to the invention, at small desired values for the axialdeflection Dax a further aspect becomes relevant. It may be appreciatedthat for realising such small deflections Dax, the pulley structure mayneed to be considerably strengthened. In fact such requiredstrengthening may become that elaborate that any increase in efficiencydue to a decrease of the radial slip of the belt is completelycounteracted by an decrease thereof due to the increased mass of thepulley to realise such small deflection Dax, or, additionally, themanufacturing cost become so high that they are no longer compensated bythe said increased efficiency. According to the invention an axialdeflection Dax of about 0.1 mm is considered a practically applicableoptimum value. From FIG. 8 it appears that at such Dax value the entirerange of cone angles λ between 5 and 11 degrees can be applied withinthe Sag range as claimed by the present invention.

According to the invention, known pulley designs may be brought intoconformity with the present invention by adopting such pulley design ina transmission wherein the maximum clamping force that is applied duringoperation is relatively low, or, alternatively, by generally stiffeningthe pulley construction, e.g. by using another, more rigid, material orby using more material, e.g. a thicker shaft or thicker discs.

The invention also provides for favourable pulley design modificationsthat enable the adaptation of the design to cope with a high maximumclamping force while enabling the torque transmission efficiency to beimproved, which modifications are considered more efficient andeffective than the above-mentioned obvious but unfavourable andexpensive measures of adding more material or using an inherentlystiffer material. Thus the invention further provides for severalmeasures that may be applied to known pulley designs so as to reduce thevalue of the Sag parameter and to allow the transmission to be operatedat an improved efficiency.

In a first measure according to the invention, a third bearing is addedaround the pulley shaft 14 having a bearing on each side of the set ofpulley discs adjacent thereto, said third bearing being fittedimmediately adjacent to the bearing of the fixed disc 15. By thismeasure bending of the shaft 14 under the influence of the said bendingforces Fb is largely reduced. Thereby the axial deflection Dax of thepulley discs 15, 16 may be greatly reduced, in some cases even halved.Alternatively, the shaft diameter 14 may be increased, however, this isgenerally not preferred in the art, since it only comes at an increasein the overall size of the transmission or a reduction of the range ofavailable transmission ratios. This measure relies on the importantinsight that the axial deflection of the fixed disc 15 will generally beconsiderably larger than that of the axially movable disc 16, becausesaid former disc 15 is only supported in axial direction where it isfixed to the shaft 14, whereas the axially movable disc experiences anaxially oriented force due to the pressure exerted in the piston of thepiston/cylinder assembly which generally extends over a large part ofthe radial dimension of the relevant disc 16, possibly even between theshaft 14 and a radially outer edge of the disc 16. Therefore,additionally supporting the fixed disc will generally be far moreeffective in reducing the maximum amount of axial deflection Dax thanimplementing the same measure at the axially movable disc.

In a second measure according to the invention illustrated in FIG. 9, atleast the fixed pulley disc 15 of a pulley 4, 5 is strengthened byincreasing its axial dimension, however, to conserve weight suchincrease is applied not over the entire tangential dimension of the disc15, but only locally for instance by incorporating a number of radiallyoriented strengthening ribs having a limited tangential dimension or byproviding recesses or holes in the surface of the disc 15 facing axiallyaway from the belt 1. Preferably such recesses or holes are mutually ina hexagonal relationship. According to the invention it is particularlyadvantageous to generally reinforce the radially inward located base 27of the disc 15, such that the axial width of the disc 15 increasesconsiderably in radial outward direction along from the said base 27 toits radially outer edge 28.

In a third and final measure according to the invention, the cone angleλ of at least the fixed pulley disc 15 is provided such that itincreases in radial outward direction between a most radial inwardposition on its belt contact surface 29 and a most radial outwardposition thereon, which is illustrated for the fixed disc 15 of a pulleyin FIG. 10. As explained in the above the cone angle λ has a largeinfluence on the Sag parameter for a given axial deflection Dax. It may,however, also be understood that the said axial deflection Dax or,alternatively, amount axially outward bending of a pulley disc 15increases with an increasing radial position on the pulley disc wheresuch deflection is determined, because of the increasing force moment ofthe reaction force Fr with respect to the pulley shaft 14. Thus byapplying a larger cone angle λ in radial outward direction on the saidcontact surface 29, the effect of the axial deflection Dax increasing insuch direction, may be counteracted to a smaller or larger extend. Tothis effect the cone angle of a pulley disc 15 increases continuously inradially outward direction, e.g. by giving the contact surface 12 asuitable arc shape having a radius of curvature R as seen in atangential cross section, as indicated in FIG. 10. Preferably, however,a spline curve is used to define the shape of the contact surface 29.According to the invention, in this respect it is considered preferableif said spline curve is chosen such that during operation the maximumvalue of the radial Sag-parameter is essentially constant for eachradial position on the pulley disc. This was found to be approximatelythe case when the cone angle is about equal to 7 degrees at the mostradial inward position on the contacting surface 29 of the pulley disc15 and gradually increases to about 11 to 12 degrees at the most radialoutward position thereon.

By designing a pulley in accordance with the invention, it will berealised that the torque transmission efficiency to be achieved inreality by a transmission design may be predicted and be consciouslycontrolled in advance in a largely reliable manner. As may be evidentfrom the present description, the single parameter Sag according to theinvention takes account of the major factors influencing suchefficiency, including mechanical stiffness of the pulley construction,the cone angle λ, and the amount clamping force Fc on the belt.

It is further remarked that the present invention is particularlyrelevant for a transmission equipped with the type of assembledtransmission belt described herein. This is caused by the fact that theelements 3 or mounted movably on the carrier so that they aredisplaceable with respect to each other to a limited extend, whereas forinstance a chain belt or a rubber band to a certain extend form acontinuous structure. Thus the behaviour and trajectory of the assembledbelt, will be influenced by the said axial deformation or deflection ofthe pulley discs to a relatively large extend.

The invention not only relates to the entirety of the precedingdescription and all details of the pertaining figures, but also to allthe features of the following claims.

1. Continuously variable transmission for motor vehicles, comprising: an endless tensile means (2); a continuous array of transverse elements (3), said tensile means (2) and said transverse elements (3) together forming a transmission belt (1); and a set pulleys (4, 5), each pulley mounted on a respective central pulley shaft (14), each pulley (4, 5) having a mechanical stiffness and comprising a fixed disc (15) axially fixed with respect to said pulley shaft (14) and a movable disc (16) axially movable, under influence of a pressure exerted on said movable disc, for clamping said belt (1) between said discs (15, 16), the exerted pressure providing a clamping force exerted on said belt (1) by said discs (15, 16) and comprising an axial force Fax exerted on said movable disc (16), each disc having a belt contact surface (29), each transverse element (3) having lateral pulley contact surfaces (6) for contacting said belt contact surface (29) of each of said discs (15, 16) and being provided slideably on said tensile means (2) of said belt (1), said belt contacting surfaces (29) mutually being oriented in a disposition with radially outward increasing mutual distance such that a cone angle λ is formed between each said belt contacting surface (29) and an imaginary radial line through said pulley shaft (14), said cone angle λ defining the angle under which said contact faces (29) and said transverse elements (3) mutually contact at each radial position of said belt (1) with respect to an axis of rotation of said pulley (4, 5), wherein a stiffness related feature of each said pulley (4, 5), when expressed as a parameter Sag that indicates an amount of radial displacement of said belt (1) between said discs (15, 16) occurring in response to imposing a maximum amount of said axial force Fax during operation of said transmission, said displacement being measured relative to an initial radial position of said belt (1) in an unloaded state of the transmission, has a value in the range between 0.5 mm and 1.2 mm.
 2. Transmission according to claim 1, wherein the Sag parameter is determined by a maximum amount of axial deflection Dax at a radial outer edge of the relevant pulley disc (15, 16) occurring in response to imposing the maximum amount of the said axial force Fax during operation of the transmission divided by the tangent of the cone angle λ of the transmission.
 3. Transmission according to claim 2, wherein the axial deflection Dax is approximated by a FEM calculation method.
 4. Transmission according to claim 2, wherein the maximum amount of axial deflection Dax at a radial outer edge of the relevant pulley disc (15, 16) occurring in response to imposing the maximum amount of the said axial force Fax during operation of the transmission is about 0.1 mm.
 5. Transmission according to claim 1, wherein the cone angle λ of the transmission is less than 11 degrees.
 6. Transmission according to claim 1, wherein the transmission is designed to and capable of transmitting at least 250 Nm.
 7. Transmission according to claim 1, wherein the cone angle λ at which the belt (1) and each said pulley (4, 5) co-operate in said transmission has a value in a range between 6 and 10 degrees.
 8. Transmission according to claim 1, wherein the pulley shaft (14) is provided with at least three bearing means (12, 13), at least two bearing means being located immediately adjacent to the fixed disc (15) of the pulley (4, 5) for limiting a bending of the shaft (14) during operation of the transmission.
 9. Transmission according to claim 1, wherein an axial width of at least the fixed disc (15) of a pulley (4, 5) increases considerably from a radially outer edge (28) thereof in a radial inward direction towards an radially inward located base (27), whereby as seen in tangential direction the said axial width varies along the circumference of the disc (15).
 10. Transmission according to claim 1, wherein at least one disc (15) of the pulley (4, 5) is provided such that the cone angle λ increases in radial outward direction along the belt contact surface (29) of the disc (15).
 11. Transmission according to claim 10, wherein the cone angle λ lies within the range between 7 and 12 degrees.
 12. Continuously variable transmission for motor vehicles, comprising: an endless tensile means (2); a continuous array of transverse elements (3), said tensile means (2) and said transverse elements (3) together forming a transmission belt (1); and a set pulleys (4, 5), each pulley mounted on a respective central pulley shaft (14), each pulley (4, 5) having a mechanical stiffness and comprising two discs (15, 16) with at least one of the two discs being a movable disc (16) axially movable, under influence of a pressure exerted on said movable disc, for clamping said belt (1) between said discs (15, 16), the exerted pressure providing a clamping force exerted on said belt (1) by said discs (15, 16) and comprising an axial force Fax exerted on said movable disc (16), each disc having a belt contact surface (29), each transverse element (3) having lateral pulley contact surfaces (6) for contacting said belt contact surface (29) of each of said discs (15, 16) and being provided slideably on said tensile means (2) of said belt (1), said belt contacting surfaces (29) mutually being oriented in a disposition with radially outward increasing mutual distance such that a cone angle λ is formed between each said belt contacting surface (29) and an imaginary radial line through said pulley shaft (14), said cone angle λ being less than 11 degrees, said cone angle λ defining the angle under which said contact faces (29) and said transverse elements (3) mutually contact at each radial position of said belt (1) with respect to an axis of rotation of said pulley (4, 5), wherein a stiffness related feature of each said pulley (4, 5), when expressed as a parameter Sag that indicates an amount of radial displacement of said belt (1) between said discs (15, 16) occurring in response to imposing a maximum amount of said axial force Fax during operation of said transmission, said displacement being measured relative to an initial radial position of said belt (1) in an unloaded state of the transmission, has a value in the range between 0.5 mm and 1.2 mm.
 13. Transmission according to claim 12, wherein the transmission is designed to and capable of transmitting at least 250 Nm.
 14. Continuously variable transmission for motor vehicles, comprising: an endless tensile means (2); a continuous array of transverse elements (3), said tensile means (2) and said transverse elements (3) together forming a transmission belt (1); and a set pulleys (4, 5), each pulley mounted on a respective central pulley shaft (14), each pulley (4, 5) having a mechanical stiffness and comprising two discs (15, 16) with at least one of the two discs being a movable disc (16) axially movable, under influence of a pressure exerted on said movable disc, for clamping said belt (1) between said discs (15, 16), the exerted pressure providing a clamping force exerted on said belt (1) by said discs (15, 16) and comprising an axial force Fax exerted on said movable disc (16), each disc having a belt contact surface (29), each transverse element (3) having lateral pulley contact surfaces (6) for contacting said belt contact surface (29) of each of said discs (15, 16) and being provided slideably on said tensile means (2) of said belt (1), said belt contacting surfaces (29) mutually being oriented in a disposition with radially outward increasing mutual distance such that a cone angle λ is formed between each said belt contacting surface (29) and an imaginary radial line through said pulley shaft (14), said cone angle λ defining the angle under which said contact faces (29) and said transverse elements (3) mutually contact at each radial position of said belt (1) with respect to an axis of rotation of said pulley (4, 5), wherein a stiffness related feature of each said pulley (4, 5), when expressed as a parameter Sag that indicates an amount of radial displacement of said belt (1) between said discs (15, 16) occurring in response to imposing a maximum amount of said axial force Fax during operation of said transmission, said displacement being measured relative to an initial radial position of said belt (1) in an unloaded state of the transmission, has a value in the range between 0.5 mm and 1.2 mm, and wherein the transmission is designed to and capable of transmitting at least 250 Nm.
 15. Transmission according to claim 14, wherein the maximum amount of axial deflection Dax at a radial outer edge of the relevant pulley disc (15, 16) occurring in response to imposing the maximum amount of the said axial force Fax during operation of the transmission is about 0.1 mm.
 16. Transmission according to claim 14, wherein the cone angle λ has a value in a range between 6 and 10 degrees.
 17. Transmission according to claim 14, wherein the cone angle λ has a value in a range between 7 and 12 degrees.
 18. Transmission according to claim 14, wherein an axial width of at least a fixed disc (15) of a pulley (4, 5) increases considerably from a radially outer edge (28) thereof in a radial inward direction towards an radially inward located base (27), whereby as seen in tangential direction the said axial width varies along the circumference of the disc (15), incorporating radially oriented strengthening ribs having a limited tangential dimension.
 19. Transmission according to claim 14, wherein at least one disc (15) of the pulley (4, 5) is provided such that the cone angle λ increases in radial outward direction along the belt contact surface (29) of the disc (15).
 20. Transmission according to claim 14, wherein the cone angle λ of the transmission is less than 11 degrees. 